When eating your favorite meal, the ocean is the last thing on your mind. But do you know that what we eat can significantly impact our oceans? Well, agriculture is one of the biggest causes of dead zones, which damages our marine life and ecosystems. So, what is a dead zone?
Read on as we look at what a dead zone is, its types, how dead zones occur, their impact, and how our oceans can recover from dead zones.
Related Read: How Much of the Ocean Has Been Explored?
A dead zone, or, in scientific terms, an oxygen minimum zone, refers to a section of the ocean characterized by severely low oxygen levels4. The existence and survival of marine life hinge heavily on the availability of an ample concentration of dissolved oxygen. This necessary oxygen is deficient in a dead zone, creating environments unfit for plant and aquatic life.
Unfortunately, the depletion of oxygen levels in the ocean leads to the death of marine life or forces them to flee, destabilizing our marine ecosystem. These dead zones aren't limited to oceans. They also appear in lakes, rivers, and ponds.
Though dead zones can form naturally, human actions, including agricultural practices and climate change impacts, contribute significantly to their
We have four main types of dead zones based on how long the hypoxia lasts. This could be for hours, days, weeks, months, and even years.
A permanent dead zone typically occurs in deep water where oxygen levels remain low throughout the year. It stays below two milligrams of dissolved oxygen per liter of water.
The temporary dead zone stays hypoxic for only a short period. Temporary dead zones could be a few hours or days.
Seasonal dead zones occur every year. It appears when warmer temperatures and increased rainfall have flushed more nutrients into the waterways.
The Diel cycling hypoxia is a type of dead zone that occurs in the warmer months but at night and early morning hours.
So, how exactly do dead zones form? Ocean dead zones occur by a process called eutrophication. This happens when excess nutrients enter into the oceans and waterways. These extra nutrients include nitrogen and phosphorus, which stimulate the growth of algal blooms.
The excessive growth of the algal bloom covers the water's surface to create dead zones. Some of these algal blooms may contain harmful toxins, which we then classify as harmful algal blooms. These toxic algae grow due to a type of algae known as blue-green algae.
The harmful algal bloom eventually dies, sinking to the bottom of the ocean floor using up dissolved oxygen from the water. The decomposition process consumes oxygen, depleting the remaining oxygen available in the water. This decomposition process may happen so fast that aquatic organisms cannot escape the low-oxygen zone, resulting in suffocation and, ultimately, death.
Elevated nutrient levels and the growth of blue-green algae can also contaminate drinking water in communities2, causing animal and human illness.
Here are a few causes of dead zones in our oceans and waterways:
Agriculture is the most significant cause of nutrient pollution, leading to the formation of dead zones. According to a study, 60% of bays and coastal waters around the United States are affected by nutrient pollution from land-based activities.
Intensive farming practices, especially in animal agriculture, lead to excess nutrients, including nitrogen and phosphorus from animal manure and chemical fertilizers, flowing into our waterways and ending up in coastal regions. As bacteria munch on animal waste, it increases carbon dioxide, producing less oxygen and harming aquatic species.
Nitrogen and phosphorus from fertilizers used to grow feed for farm animals are primarily responsible for the Gulf of Mexico dead zone, which is the second-largest dead zone in the world.
Agriculture isn’t the only source of nitrogen. Cars and other vehicles also release significant amounts of nitrogen and phosphorus into the atmosphere. These vehicular gas emissions can enter coastal waters, distorting the health of our oceans to create dead zones.
Factory farms and power plants that burn fossil fuels to generate electricity are sources of nitrogen and phosphorus gases, which can penetrate our waters through the atmosphere.
Temperature changes, storm patterns, rising wind, rain and sea levels, ocean acidification, and other climate change variables can contribute to spreading dead zones globally. Increased levels of carbon dioxide and warmer temperatures promote the growth of algal blooms.
As sea temperature levels rise, the warmer waters hold less oxygen dissolved, leading to the easy formation of dead zones3.
A dead zone can also occur seasonally as the mixing of the water column and other factors change. For example, dead zones start forming in the Gulf of Mexico during early spring and then spread during the fall as the water column mixes.
Human activity is a major cause of dead zones. However, a dead zone may also occur naturally due to some chemical, physical, and biological processes. For example, seasonal current upwelling can lead to hypoxic conditions in deep waters with excess nutrients and less oxygen.
Hypoxic zones can affect the health of our oceans, the marine mammals and shore birds that live in and near them, and the people who depend on them for survival. Let’s glimpse at the environmental and economic impacts of an ocean dead zone:
As algae grows and dies, the decomposition process consumes oxygen, causing marine organisms to die. Typically, only a few organisms can swim out of the dead zone. Immobile species like oysters and mussels that need more oxygen to survive will slowly die due to low oxygen, displacing many fish species and disrupting marine food webs.
Studies also link hypoxic zones in the ocean to reduced gonadal development, reduced sperm and egg quality, and low survival of fish larvae.
Low oxygen levels in the oceans emit nitrogen, nitrous oxide, methane, C02, and other harmful greenhouse gases contributing to climate change.
A dead zone doesn’t only affect wildlife but also impacts the economy.
Fishermen who live on these seas must venture farther from these low-oxygen areas to find fish. Consequently, the increase in travel distance escalates their transportation expenses. This journey may not be feasible for fishermen with smaller vessels.
Again, a dead zone can affect aquatic life, reducing the availability of commercial fish species like shrimps.
For example, the dead zone in The Gulf of Mexico has increased the prices of larger brown shrimps as they are not commonly caught in a hypoxic zone compared to smaller shrimps.
The hypoxic zone in our ocean also makes it unsafe for swimming. It impacts businesses in coastal areas, including hotels and restaurants.
A hypoxic zone in salt and freshwater and the Great Lakes can occur worldwide. However, they are more common in coastal areas near watersheds.
The number of dead zones can vary from year to year. Today, we have many hypoxic zones worldwide, which continue to increase. Here are some of the biggest dead zones in the world:
Over the years, dead zones have multiplied. But it isn’t all bad news. If proper actions are taken, some dead zones, especially those resulting from human activity like pollution, can be recovered.
Although dead zones may occur naturally, human activity, like nutrient loading during agricultural practices, has the most impact on forming dead zones.
These dead zones signal global disaster for our environment. They are a financial threat to fishermen who rely on the ocean for their livelihood. In the face of a climate and economic crisis, taking immediate action has never been more critical.
| 1 | Queste, B. Y., Vic, C., Heywood, K. J., & Piontkovski, S. A. (2018). Physical controls on oxygen distribution and denitrification potential in the north west Arabian Sea. |
| 2 | CDC. (2022). Facts About Cyanobacterial Blooms for Poison Center Professionals | CDC. |
| 3 | Altieri, A.H. and Gedan, K.B. (2015), Climate change and dead zones. |
| 4 | NOAA. (n.d.). What Is a Dead Zone? |
Jen’s a passionate environmentalist and sustainability expert. With a science degree from Babcock University Jen loves applying her research skills to craft editorial that connects with our global changemaker and readership audiences centered around topics including zero waste, sustainability, climate change, and biodiversity.
Elsewhere Jen’s interests include the role that future technology and data have in helping us solve some of the planet’s biggest challenges.
Fact Checked By:
Isabela Sedano, BEng.
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